The concentration of carbon contained in a steel should be very low, for example, in the case of a thin steel sheet for use in automobiles and a thin steel sheet for making beverage cans, to improve the workability thereof and prevent an increase of the deep drawing resistance derived from aging.
In general, in the iron industry, a carbon removal treatment (i.e., a decarbonization treatment) is conducted through the use of various vacuum or reduced pressure decarbonization units as described in, for example, "Tekko Binran II-Seisen.Seiko (Handbook of Steel-Pig Iron Making. Steel Making)", 3rd edition, pp. 671-685.
The above-described decarbonization treatment of a molten steel is conducted by removing carbon [C] contained in a molten steel by the following reaction, through the use of oxygen [O] incorporated in a molten steel and various oxidization sources such as iron ore Fe.sub.x O.sub.y and oxygen gas O.sub.2. EQU [C]+[O]=CO(gas) EQU y[C]+Fe.sub.x O.sub.y =yCO(gas)+xFe (1) EQU [C]+1/2O.sub.2 =CO(gas)
Nevertheless, even in the case of vacuum or reduced pressure equipment, the decarbonization rate begins to fall when the carbon content [% C] of the molten steel is 0.015% by weight or less, and further falls to a carbon content [% C] of about 0.005% by weight. Accordingly, to manufacture a low carbon molten steel, the decarbonization treatment time should be prolonged, which lowers the molten steel temperature. Therefore, in general, to compensate for the lowering of the molten steel temperature, the molten steel is reheated in the next step, or the tapping temperature in the previous step is increased. Nevertheless, when the tapping temperature is high, the refractory is subjected to melt loss, and this increases the refractory unit requirements and the cost of the decarbonization treatment.
Thus, even when use is made of vacuum or reduced pressure equipment, the current decarbonization treatment still has problems of efficiency and profitability, and therefore, it is obvious that no practical method exists wherein a molten steel is decarbonized to the above-described carbon content or less under atmospheric pressure.
The denitrogenization-dehydrogenation reaction of a molten metal is also conducted through the utilization of a reduced pressure or vacuum, based on the following reaction formulae: EQU [N]+[N]=N.sub.2 (gas) (2) EQU [H]+[H]=H.sub.2 (gas) (3)
Nevertheless, the development of a method of manufacturing a molten metal having lower nitrogen/or hydrogen contents, with a high efficiency, is still unknown in the art.
To overcome the above problems of the conventional method, a gas bubbling or gas injection by an inert gas is conducted to increase the area of the gas-liquid reaction interface, and at the same time, the flow rate of the blown gas is increased to enhance the stirring of the molten steel and thereby increase the mass transfer rate of [C] (and [N]), to thus increase the removal reaction rate. Under a reduced pressure or vacuum, the increases in the blown gas flow rate makes it impossible to ensure that the gas-liquid interface area becomes a reaction site, and increases the amount of scattering of the molten steel due to a coalescence of blown gases or blow-through of blown gases, etc., and the molten steel flies out from the container accommodating the molten steel (hereinafter referred to as "ladle") or a layer of metal is deposited on the internal wall of the ladle, which makes it difficult to simultaneously attain the desired reaction rate and a stable refining procedure.
To manufacture a ultra-low carbon steel (and a ultra-low nitrogen steel), it is necessary to prolong the time taken for decarbonization (and denitrogenation) refining, even when this incurs the above-described difficulties. Accordingly, in the prior art method, to compensate for the lowering of the molten steel temperature, the molten steel is reheated in the next step, or a high temperature is used as the temperature of the molten steel tapped from a converter or an electric furnace. When the tapping temperature is high, the refractory of the convertor or the electric furnace is subjected to melting loss, which increases the refractory unit requirements, and accordingly, the cost of the refining. Thus, even when use is made of vacuum or reduced pressure equipment, the existing decarbonization-denitrogenation treatment method is inefficient and nonprofitable to an extent such that it is very difficult to stably manufacture a molten steel having a ultra-low nitrogen content and a ultra-low nitrogen content in a short time.
Further, Japanese Unexamined Patent Publication (Kokai) No. 62 62-156220 discloses the acceleration of a slag/metal reaction in the desulfurization of a molten metal with slag, although this is not related to the decabonization, denitrogenation or dehydrogenation treatment of a molten metal or a molten alloy. The slag/metal reaction disclosed in this document is a liquid-liquid reaction, and impurities to be removed from the molten metal are captured in the slag. Therefore, an increase in the slag/metal contact area increases the rate of removal of the impurities. One means of causing the turbulence of slag/metal is to blow a gas around the interface of the slag and the molten metal, and thus the presence of a slag is indispensable to the method of the present invention.
The following method of the present invention intends to accelerate the degassing reaction (gas-liquid reaction), for example, to allow CO, N.sub.2 and H.sub.2 formed by the reaction to be absorbed with the blown gas, and is essentially different from the invention of the above-described publication, in the reaction site thereof. Therefore, the turbulence of slag/metal by blowing a gas to increase the slag/metal contact area does not accelerate the degassing reaction, and thus the presence of the slag is not essential but is a useful means of increasing the molten metal/gas contact area.